The present invention relates to a method of reacting natural gas and oxidant under conditions to optimize the formation of desired alkyl oxygenates (especially, methanol). More specifically, the embodiments are for reacting a C1-C4 alkane (methane, ethane, propane, and butane) into an alkyl oxygenate, and, more particularly in a focal application, for direct oxidation (under partial oxidation conditions) conversion of methane into methanol.
The current industrial practice for methanol production is a two-step, Fischer-Tropsch type chemical process. The first step is the endothermic reforming of methane from natural gas to carbon monoxide and hydrogen, followed by a second step consisting of a solid-catalyzed reaction between carbon monoxide and hydrogen to form methanol. This technology is energy intensive and the process economics are unfavorable for all but very large scale methanol plants.
Various methods and apparatuses for the conversion of methane into methanol are known. It is known to carry out a vapor-phase conversion of methane into a synthesis gas (mixture of CO and H2) with its subsequent catalytic conversion into methanol as disclosed, for example, in Karavaev M. M., Leonov B. E., et al “Technology of Synthetic Methanol”, Moscow, “Chemistry” 1984, pages 72-125. However, in order to realize this process it is necessary to provide complicated equipment, to satisfy high requirements for the purity of the gas, to spend high quantities of energy for obtaining the synthesis gas and for its purification, and to have a significant number of intermittent stages from the process. Also, for medium and small enterprises with the capacity of less than 2,000 tons/day it is not economically feasible.
Russian Patent No. 2,162,460 includes a source of hydrocarbon-containing gas, a compressor and a heater for compression and heating of the gas, and a source of oxygen-containing gas with a compressor. It further includes successively arranged reactors with alternating mixing and reaction zones and a means to supply the hydrocarbon-containing gas into a first mixing zone of the reactor and the oxygen-containing gas into each mixing zone, a recuperative heat exchanger for cooling of the reaction mixture through a wall by a stream of cold hydrocarbon-containing gas of the heated hydrocarbon-containing gas into a heater, a cooler-condenser, a partial condenser for separation of waste gases and liquid products with a subsequent separation of methanol, a pipeline for supply of the waste gas into the initial hydrocarbon-containing gas, and a pipeline for supply of waste oxygen-containing products into the first mixing zone of the reactor.
In this apparatus, however, fast withdrawal of heat from the highly exothermic oxidation reaction of the hydrocarbon-containing gas in not achievable because of the inherent limitations of the heat exchanger. This leads to the need for reduction in the quantity of supplied hydrocarbon-containing gas and, further, it reduces the degree of conversion of the hydrocarbon-containing gas. Moreover, even with the use of oxygen as an oxidizer, it is not possible to provide an efficient recirculation of the hydrocarbon-containing gas due to the rapid increase of the concentration of carbon oxides. A significant part of the supplied oxygen is wasted for oxidation of CO into CO2, and thereby additionally reduces the degree of conversion of the initial hydrocarbon-containing gas to useful products and provides a further overheating of the reaction mixture. The apparatus also requires burning an additional quantity of the initial hydrocarbon-containing gas in order to provide the utility needs of a rectification of liquid products. Since it is necessary to cool the gas-liquid mixture after each reactor for separation of liquid products and subsequent heating before a next reactor, the apparatus is substantially complicated and the number of units is increased.
A further method and apparatus for producing methanol is disclosed in the patent document RU 2,200,731, in which compressed heated hydrocarbon-containing gas and compressed oxygen-containing gas are introduced into mixing zones of successively arranged reactors, and the reaction is performed with a controlled heat pick-up by cooling of the reaction mixture with water condensate so that steam is obtained, and a degree of cooling of the reaction mixture is regulated by parameters of escaping steam, which is used in liquid product rectification stage.
Other patent documents such as U.S. Pat. Nos. 2,196,188; 2,722,553; 4,152,407; 4,243,613; 4,530,826; 5,177,279; 5,959,168 and International Publication WO 96/06901 disclose further solutions for transformation of hydrocarbons.
There is also a need for a one step process that is also suitable for small-scale processing, overcoming process scale limitations of the Fischer Tropsch method, and also making “stranded gas” a valuable commodity. This approach makes use of a homogeneous, gas phase partial oxidation reaction, carried out by contacting natural gas and an oxidant, with the oxidant as the limiting reagent. The most abundant products are methanol and formaldehyde, coming from methane, the principal component of natural gas. Smaller amounts of ethanol and other oxygenated organic compounds are formed by oxidation of methane, ethane, propane, and higher hydrocarbons that are all minor constituents of natural gas. These reaction products are all liquids, and are transportable to a central location for separation and/or subsequent use as fuels or as chemical intermediates. A central feature of such processes is that the process chemistry can be executed in the field at remote locations.
U.S. Pat. No. 4,618,732 (“Direct conversion of natural gas to methanol by controlled oxidation” to Gesser, et al.) describes a process for converting natural gas to methanol. The selectivity for methanol is indicated as resulting from careful premixing of methane and oxygen along with the use of glass-lined reactors to minimize interactions with the processing equipment during the reaction. The need for mixing prior to entering a reactor for reaction initiation is indicated in the following extract:
“The mixing of gases preferably takes place in a pre-mixing chamber or “cross” of relatively small volume and then pass through a short pre-reactor section before entering the heated reaction zone. However, when mixing gases at high pressure in a relatively small volume, laminar flow often takes place with the oxygen or air forming a narrow homogeneous stream within the general flow of natural gas. The oxygen or air has little chance of becoming dispersed throughout the reaction stream prior to reaching the reaction zone. Without wishing to be bound by theory, when this takes place it is postulated that the natural gas is oxidized initially to methanol which is further oxidized, at the periphery of the oxygen stream, i.e. in an oxygen-rich environment, to higher oxidation products.”
U.S. Pat. No. 4,618,732 to Gesser also emphasizes the need to keep the reaction from initiation until mixing is completed (—“mixing oxygen and natural gas prior to their introduction into a reactor”).
U.S. Pat. No. 4,982,023 (“Oxidation of methane to methanol” to Han, et al.) brings forth that a plurality of reactions is occurring in the direct oxygenation of methane to methanol. In this regard, U.S. Pat. No. 4,982,023 indicates some consideration of reaction-kinetics issues in the discussion of that patent's subject matter:
“The mechanism of methanol formation is believed to involve the methylperoxy radical (CH3OO) which abstracts hydrogen from methane. Unfortunately, up until now, the per pass yields have been limited. This limited yield has been rationalized as resulting from the low reactivity of the C—H bonds in methane vis-a-vis the higher reactivity of the primary oxygenated product, methanol, which results in selective formation of the deep oxidation products CO and CO2 when attempts are made to increase conversion.”
U.S. Pat. No. 4,982,023 also makes it clear that methane and oxygen are to be premixed prior to reaction as noted in the following extract: “ . . . natural gas and the oxygen or air are kept separate until mixed just prior to being introduced into the reactor. However, if desired, the oxygen and natural gas may be premixed and stored together prior to the reaction”.
Unfortunately, laboratory results regarding methanol selectivity and single pass yield for non-catalyzed direct oxygenation of methane to methanol have not been reliably duplicated in scaling the reaction technology to manufacturing-sized systems. The need for an efficient and low cost method for enabling non-catalyzed direct oxygenation of methane to methanol continues to be unfulfilled.